I believe the bare edison module is (while being much weaker overall) even more expensive than the RasPi 3 (which is after all a complete board with connectors and power supply...).

The Edison mostly has one advantage over the Pi - it's a lot smaller. Great for IoT stuff, but I don't think it would be particularly useful to control a 3D printer - at least not in it's current state.

/edit:
Confirmed, the Edison module starts at about 55$ on mouser.com (with different versions available) while the RasPi 3 is still sitting at a beautiful 35$ selling price.

Haven't heard of any. Like mentioned before, the Pi isn't suitable as a direct controller - because it doesn't have a real-time OS. Even my own platform will "only" use the Pi to shove the gcode data into the FPGA which is the actual real-time controller. (this is simplified of course. The Pi does in fact do more than just that, but it doesn't do any of the control tasks itself)

I actually see machinekit as very interesting as it is a fork of LinuxCNC that are rewriting stuff to be able to run on embedded systems and get better separation of backend/frontend.

To me most of the controller boards are totally uninteresting for CNC use, since i cannot hookup a monitor and a control panel, where i can see DRO, use MDI mode, preview my gcode etc. It also got a very nice hardware abstraction layer and a software PLC to program event chains for toolchangers, automatic tool measuring etc. Fire-and-forget does not work in the same way for a CNC mill as for a 3D Printer, i do not get people that just run GRBL on a arduino and run a gcode file without any feedback, that is scary stuff...

True, I would never run a CNC on something as crude as a 3D printer controller
On that note - I'm going to use an EdingCNC controller for my CNC build (which may or may not be finished by the end of this year )